Some conventional LED drivers use PWM and related control techniques to deliver current to LEDs. The PWM technique is a common method to control gradient levels of frame content while rendering the frame content to control a grayscale in modern display electronic circuits. PWM is increasingly used in modern commercial LED driver integrated circuits to deliver pulsed and controlled mean current to the LEDs in most high pitch large format Direct View LED (DV-LED) displays.
An LED display panel generally refers to a device which comprises an array of LEDs that are arranged in one or more rows and columns. Alternatively, an LED display panel may include a plurality of sub-modules, each sub-module having one or more such LED arrays. LED panels may employ arrays of LEDs of a single color or different colors. When LEDs of the same color are used in certain display applications, each LED normally corresponds to a display unit or pixel. When LED panels employ LEDs of different colors for a full-color display, a display unit or pixel normally includes a cluster of three LEDs—typically a red LED, a green LED, and a blue LED. Such a cluster of three LEDs may be referred to as an RGB unit.
An LED driver circuit delivers power to the array of LEDs and controls the current delivered to the array of LEDs. The LED driver circuit may be a single channel driver or a multi-channel driver. Each channel of the driver circuit may deliver power to a plurality of LEDs and control the current delivered to the LEDs. When a group of LEDs is electrically coupled to the same channel, the group of LEDs are often referred to as a “scan line.”
In general, LED driver circuits control the brightness of the LEDs by varying the current delivered to and flowed through the LEDs. In response to the delivered current, the LED emits light with a brightness in accordance with the characteristic specifications of the LED. A greater current delivered to the LED usually translates to a greater intensity of brightness. To effectively control the delivery of current, LED driver circuits may employ a constant current source in combination with the modulation (i.e., turning ON and OFF) of the constant current source, using, for example, PWM to achieve a desired average (mean) current over each scan cycle.
Dithering is a technique that aims to achieve a gradient using an insertion of a number of intermediate colors when abrupt color transitions are seen in content. Color artists use this technique to modify content where visible step transitions in a color gradient due to limited color resolution cause an artifact referred to as banding. Dithering has been used in early machine and rendering devices that were too primitive to display more than a few colors. The reason dithering is effective is because the human eye is imperfect and can distinguish the pixels with limited accuracy and resolution, so the human eye tends to mix the color of a specific pixel with the pixels' neighboring pixels. PWM dithering exploits these properties of the human eye to create an appearance of smoother color gradient, by selectively adding noise at abrupt color transitions.
There are a variety of known PWM based solutions and architectures deployed in the design of modern LED drivers and some of these solutions and architectures use dithering in conjunction with PWM. The present inventor has recognized that known PWM dithering solutions are not effective when the brightness of the content is too high or too low as PWM dithering adjustments are applied uniformly to all the frame content without consideration of brightness levels of the frame content.